The present invention relates to a method of operating a hydrogen dispensing unit. The present invention more particularly relates to a method of operating a hydrogen dispensing unit where the hydrogen is cooled prior to dispensing the hydrogen to a hydrogen storage tank in a vehicle.
Hydrogen dispensing units are used to dispense high pressure hydrogen into hydrogen storage tanks in vehicles such as cars, buses, trucks, and forklifts.
There is growing interest to use hydrogen as a transportation fuel in cars, buses, trucks, and other vehicles. Hydrogen is generally stored in a fuel tank on-board the vehicles at high pressure. After most of the on-board hydrogen has been depleted, the pressure of the hydrogen in the fuel tank is reduced and the fuel tank must be refueled with hydrogen.
During refueling, hydrogen is dispensed into the fuel tank at a hydrogen dispensing station. The dispensing station includes a hydrogen supply, which can be one or more high pressure storage tanks. Hydrogen is transferred from the high pressure storage tank into the fuel tank of the vehicle. The driving force for transferring hydrogen is the pressure difference between the high pressure storage tank and the vehicle fuel tank.
Dispensing from a high pressure supply vessel to the lower pressure receiving vessel in a vehicle results in a temperature increase of the hydrogen that was dispensed. To compensate for the temperature rise of the dispensed hydrogen, hydrogen dispensing stations may include one or more heat exchangers to cool the hydrogen as it is being dispensed. Cooling may be provided by a refrigerant in a refrigeration cycle. The heat exchanger may comprise one or more high thermal capacity cooling blocks, which are cooled by the refrigerant and through which the hydrogen passes and is cooled.
The transfer line between the high pressure storage tank and the fuel tank typically includes various control valves, and block and bleed valves. A block valve blocks the flow from the high pressure storage tank and a bleed valve allows a portion of the hydrogen trapped between the block valve and the dispensing nozzle to discharge, thereby reducing the pressure at the dispensing nozzle. Accepted standards, such as SAE J2600 and ISO 17268, require that the pressure at the nozzle be less than 0.5 MPa (gauge) before the dispensing nozzle can be disconnected from the fueling receptacle on the vehicle.
While the transfer line between the block valve and the dispensing nozzle will be at a lower pressure after dispensing hydrogen to a vehicle, the residual hydrogen trapped in the transfer lines between the control valve and one or more block valves will still be at high pressure and a cold temperature because of being cooled in the heat exchanger.
As the hydrogen dispension unit sits idle waiting to refuel another vehicle, the temperature of the residual trapped hydrogen will increase resulting in a pressure increase in the lines. The resulting pressure increase may be greater than design pressure limits for the equipment. The resulting high pressure may cause the dispensing unit to trip because of protections configured to protect the vehicle from excessive pressure, or because the excessive pressure might be interpreted by the controller as some sort of pressure transducer failure. Pressure relief valves could be used to relieve the pressure when it exceeds safe limits, but pressure relief valves are known for failing to completely reseal after relieving the pressure. Use of pressure relief valves on a frequent basis is undesirable.
Industry desires reliable hydrogen dispensing units.